The present invention relates to the production of powders by a precipitation reaction.
Various industrially important powders are produced by precipitation reactions. Thus for example, copper oxalate may be produced by reaction of a solution containing copper ions with one containing oxalate ions. A further example is a mixed yttrium and barium oxalate which may be precipitated by reaction of a solution containing yttrium and barium ions with one containing oxalate ions.
Such precipitation processes are frequently conducted using batch reactors. However a problem exists with batch reactors in that it is difficult to produce a good compositional homogeneity and the problem becomes more severe as the size of the volume of the reactor is increased for the purposes of xe2x80x9cscaling-upxe2x80x9d. As a result, the product obtained may have a wide range of particle sizes and/or non-uniform particle morphologies.
It is also known to effect such precipitation reactions in continuous flow reactors but parabolic flow profiles are generated therein resulting in similar disadvantages to those mentioned in relation to batch reactors.
It is therefore an object of the invention to obviate or mitigate the abovementioned disadvantages.
According to the present invention there is provided a process for the production of powders by precipitation from a liquid reaction mixture, the method comprising passing along a tubular reactor a segmented reaction flow comprised of discrete volumes of the reaction mixture separated by discrete volumes of a separating fluid which is substantially immiscible with said reaction mixture, the residence time of said discrete volumes of reaction mixture in the reactor being sufficient for the precipitation reaction to be effected.
In accordance with the invention, therefore, the precipitation reaction is effected by sub-dividing the reaction mixture into a plurality of discrete volumes or segments which are passed, preferably under a plug flow conditions along a tubular reactor separated by discrete volumes of a separating fluid which is immiscible with the reaction mixture. As such, a plurality of individual, and separate, volumes of the reaction mixture pass along the tubular reactor. Within each volume of the reaction mixture, the conditions for the precipitation reaction are substantially identical so that a uniform product is obtained from each volume of the reaction mixture.
In this respect, it is particularly preferred that
(i) the individual volumes of the reaction mixture are of similar (and ideally equal) size; and
(ii) the individual volumes of the separating fluid are of similar (and ideally equal) size (although not necessarily the same size as that of the reaction mixture).
This ensures that the residence time for all volumes of the reaction mixture in the tubular reactor is substantially the same to ensure uniform reaction conditions in each such batch.
It is a requirement of the separating fluid that it is substantially immiscible with the reaction mixture (at least under the conditions prevailing in the tubular reactor) to ensure that a segmented flow comprised of a plurality of discrete volumes of the reaction mixture (separated by discrete volumes of the separating fluid) may pass along the tubular reactor. It should also be ensured that the separating fluid is non-reactive towards the reaction mixture (at least under the conditions of the tubular reactor). Provided these conditions are satisfied, the separating fluid may be a gas such a air, nitrogen, oxygen, a rare gas (e.g. argon) carbon dioxide or hydrogen. Alternatively the separating fluid may be a liquid, e.g. an alkane, a petroleum derivative (e.g. kerosene), liquid paraffin, oil or silicone oil.
Depending on the particular precipitation reaction, the segmented flow may be produced either by
(i) pre-mixing the reactants and then segmenting a stream of the resultant mixture with the separating fluid or
(ii) segmenting a stream of one liquid reactant with the separating fluid thereby producing a precursor segmented reaction flow and then admixing the liquid reactant portion thereof with the remaining reactant(s) to produce the final reaction segmented flow.
The procedure (i) may, in particular, be used where the precipitation reaction is initiated by an external stimulus, e.g. heat or light. Procedure (ii) will generally be appropriate when the precipitation reaction proceeds relatively quickly after admixing of the reactants. In either case, it is highly desirable that the reactants are thoroughly admixed together.
The segmented flow may be produced from continuous streams of the liquid reaction mixture (or a liquid component thereof) and the separating fluid immiscible therewith by passing said streams along separate, respective conduits to a common segmenting reaction. The configuration of this region, and the flow rates of said streams, are set such that discrete volumes of the reaction mixture (or liquid component thereof) and the separating fluid alternately enter, and occupy the cross-section of, the segmenting region so that a segmented flow exits the outlet thereof. By way of a simple (and non-limiting) explanation as to the way in which the segmented flow may be produced, consider that the segmented fluid is a gas. In this case, it may consider that the liquid reaction mixture (or liquid component thereof) is able to pass along the segmenting region but periodically a bubble of the gas enters, and occupies the cross-section of, the segmenting region thus interrupting liquid flow therealong and thus producing the desired segmented flow. Similar considerations apply to the case where the segmenting fluid is a liquid. For any particular arrangement of segmenting region, it is a relatively simple matter to adjust the flows thereto so as to produce a segmented flow in the manner described.
The method of the invention may be applied to the production of inorganic or organic powders. The invention may be used, for example, for preparing oxalates (such as a mixed oxalate) of an alkali, alkaline earth and/or transition metal by (co-) precipitation in aqueous or alcohol medium. Alternatively the invention may be used to produce a sulfide or a mixed sulfide of at least one transition metal. Such a sulfide may be produced, for example, from a solution of the transition metal ion(s) and thioacetamide, the solution being heated to generate the precipitating anion. Further possibilities are the synthesis of oxides, mixed oxides, carbonates, mixed carbonates, hydroxides or hydroxycarbonates by precipitation or coprecipitation in aqueous or alcoholic medium in the presence of urea which is heated to generate the precipitating anion.